A speakerphone generally includes a microphone coupled to an input of a transmit signal channel and a speaker coupled to an output of a receive signal channel. A speakerphone permits "hands-free" operation of a telephone set. In a half-duplex speakerphone only one of the signal channels is active, the other channel being disabled by having a significant amount of signal loss inserted therein. In general, circuitry must determine which channel is conveying speech signals so as to disable the other channel. This determination is preferably accomplished in a rapid manner so as not to clip or otherwise attenuate the initial portion of the speech signal. This determination must also accommodate a verbal interruption of the speaking party by the silent party so as to rapidly switch between the receive and the transmit signal channels or between the transmit and the receive signal channels.
Complicating this determination are the presence of circuit and trunk noise signals and also the presence of echo signals. Echo signals may result from inadvertent acoustic coupling from the speaker to the microphone. Echo signals may also result from impedance mismatches within the audio coupling devices which connect the transmit and receive channels to the line. The echo component due to acoustic coupling between speaker and microphone may be especially difficult to control in that the degree of coupling is often influenced by such factors as the acoustic characteristics of the room, the relative position of the speaker to the microphone, or even the position of the user's body relative to the speakerphone. In that the magnitude of the echo signal component may exceed the value of the voice signal, a significant variability of speakerphone performance may be experienced.
It is therefore one object of the invention to provide a half-duplex speakerphone which switches rapidly between speech signal paths.
It is another object of the invention to provide a half-duplex speakerphone which readily provides for improved interruptability.
It is still another object of the invention to provide a half-duplex speakerphone which provides a significant immunity to the effects of circuit and trunk noise and also echo signals.